1. Field of the Invention
The present invention relates to the manufacturing, in monolithic form, of image sensors intended to be used in image capture devices such as, for example, cameras, camcorders, digital microscopes, or again digital photographic cameras.
2. Discussion of the Related Art
FIG. 1 partially illustrates a cell of an image sensor array. With each array cell are associated a precharge device and a read device. The precharge device is formed of an N-channel MOS transistor M1, disposed between a supply rail Vdd and a node I. The gate of precharge transistor M1 is capable of receiving a precharge control signal Rs. The read device is formed of the series connection of two N-channel MOS transistors. The drain of a first one of these read transistors, called M2 hereafter, is connected to supply rail Vdd. The source of second read transistor M3 is connected to input terminal P of an electronic processing circuit. The gate of first read transistor M2 is connected to node I. The gate of second read transistor M3 is capable of receiving a read signal Rd. The cell comprises a photodiode PHD. Node I is associated with a charge storage diode DS. The anode of diodes PHD and DS is connected to a reference supply rail or ground of the circuit GND. The cathode of diode DS is directly connected to node I. The cathode of photodiode PHD is connected to node I by a charge transfer N-channel MOS transistor MD. The gate of transfer transistor MD is capable of receiving a charge transfer control signal T.
The operation of this circuit will now be described. A photodetection cycle starts with a precharge phase during which a reference voltage level is imposed to diode DS. This precharge is performed by turning off second read transistor M3 and by turning on precharge transistor M1. Once the precharge has been performed, precharge transistor M1 is turned off. Then, the system is maintained as such, all transistors being off. A given time after the end of the precharge, the state at node I, that is, the real reference charge state of diode DS, is read. To evaluate the charge state, second read transistor M3 is turned on for a very short time. The cycle carries on with a transfer to node I of the photogenerated charges, that is, the charges created and stored in the presence of radiation, in photodiode PHD. This transfer is performed by turning on transfer transistor MD. Once the transfer is over, transistor MD is turned off and photodiode PHD starts photogenerating and storing charges which will be subsequently transferred to node I again. Simultaneously, at the end of the transfer, the new charge state of diode DS is read. The output signal transmitted to terminal P then depends on the pinch of the channel of first read transistor M2, which directly depends on the charge stored in the photodiode.
In practice, a cell comprises several photodiodes which are each connected to node I via an associated transfer transistor. The previously-described read method is then successively carried out for each photodiode in the cell.
FIG. 2 shows a partial simplified top view of an example of a cell 10 of an image sensor made in monolithic form. A full line shows the limit of an active N-type or P-type doped silicon area or a polysilicon portion. A dotted line shows the limit of an active doped silicon area located under a polysilicon portion.
Cell 10 comprises four photodiodes PHD1 to PHD4, each photodiode being connected to node I by an associated transfer transistor MD1 to MD4. To the left and to the right of cell 10, photodiodes PHD5 to PHD8 associated with cells adjacent to cell 10 and the associated transfer transistors MD5 to MD8 have been shown. Each photodiode PHD1 to PHD4 is formed at the level of an active area 12, for example, of a first conductivity type, for example, type N, at the level of which is also formed the source region of transfer transistor MD1 to MD4. Each active area 12 extends in an active area 14 of the opposite conductivity type, for example, type P, which corresponds to the channel of transfer transistor MD1 to MD4. An active N-type region 16 connects the four P-type active areas 14 and corresponds to the drain regions of transfer transistors MD1 to MD4, to node I and to the source region of transistor M1. Active area 16 extends successively into a P-type active area 18 and an N-type active area 20. Active areas 18 and 20 respectively correspond to the channel and to the drain region of transistor M1. Cell 10 comprises an N-type active area 22 which successively extends into a P-type active area 24, an N-type active area 26, a P-type active area 28, and an N-type active area 30. Active areas 22, 24, 26, 28, and 30 successively correspond to the source region of transistor M3, to the channel of transistor M3, to the channel of transistor M3, to the drain region of transistor M3 (and to the source region of transistor M2), to the channel of transistor M2, and to the drain region of transistor M2. For each photodiode PHD1 to PHD4, cell 10 comprises a polysilicon portion 32 at the level of which is formed the gate of transfer transistor MD1 to MD4. Cell 10 also comprises three polysilicon portions 34, 36, 38 at the level of which are respectively formed the gates of transistors M1, M2, and M3. Portion 36 is connected to node I by metal portions not shown. According to such an arrangement of the photodetector cells, groups of four adjacent photodiodes belonging to two different cells are obtained, the groups being separated by MOS transistors for charging and reading from a cell.
The color detection is obtained by associating with each photodiode of a cell a colored filter, not shown, which only lets through the light rays having a wavelength within a given range. Three types of filters corresponding to the three primary colors (red, green, blue) are generally obtained. An example of distribution of the colored filters corresponds to the Bayer pattern according to which for each group of four adjacent photodiodes, for example, PHD1, PHD2, PHD5, and PHD6, green filters are associated with photodiodes PHD1 and PHD6, a blue filter is associated with photodiode PHD5, and a red filter is associated with photodiode PHD2. Similarly, for the groups of four adjacent photodiodes PHD3, PHD7, PHD4, and PHD8, green filters are associated with photodiodes PHD7 and PHD4, a blue filter is associated with photodiode PHD3, and a red filter is associated with photodiode PHD8. Such a pattern is reproduced for all the image sensor cells.
FIG. 3 is a partial simplified cross-section view of FIG. 2 along line III-III and illustrates an embodiment in monolithic form of photodiodes PHD5, PHD1, PHD4, and PHD8. The polysilicon portions of FIG. 2 are not shown. The photodiodes are formed in a same active area of a semiconductor region 50 of a first conductivity type, for example, lightly P-type doped (P−−). Substrate 50 for example corresponds to an epitaxial layer on heavily-doped P-type silicon wafer 52 (P++). The active areas associated with photodiodes PHD5, PHD1, PHD4, and PHD8 are delimited by field insulation areas 54, for example, made of silicon oxide (SiO2). Each photodiode comprises an active region 56 of the opposite conductivity type, for example, N. Active region 56 is interposed between an overlying heavily-doped P-type region 58 (P+) and an underlying P-type region 60 (P−), more heavily doped than substrate 50 but less heavily doped than region 58. Between the field insulation region 54 located to the right of photodiode PHD1 and the field insulation region 54 located to the left of photodiode PHD4, a heavily-doped N-type active area 62 (N+) and an underlying P-type region 64 (P−) are provided. Around each field insulation region 54 is provided a heavily-doped P-type region 66 (P+) enabling connecting region 58 to the reference voltage of the cell via substrate 50.
In operation, the heavily-doped P-type regions 58, 66, and 52 are substantially permanently maintained at the reference voltage or ground, for example, 0 V. In the absence of light, active region 56 of each photodiode reaches a so-called depletion quiescent level (positive) set by the features of the diode. Active region 56 then forms of potential well which fills according to the photodiode lighting, causing a decrease in the voltage of region 56. Indeed, when photons enter a photodiode, they cause the forming of electron-hole pairs. The holes are carried off by wafer 52 while the electrons are attracted by the potential well present at the level of region 56. Each photodiode PDH5, PHD1, PHD4, and PHD8 is of so-called totally depleted type to suppress any noise at the photodiode level. For this purpose, the doping profiles are selected so that active region 56, pinched between surface region 58 and underlying region 60, is depleted. The potential in depletion regime, that is, in the absence of a radiation, is adjusted by the dopings of regions 56, 58, and 60 only.
Generally, the photodiodes have an identical structure which is optimized to have the best quantic output independently from the wavelength of the light rays which reach the photodiodes. The photons having a wavelength corresponding to blue, to green, and to red being absorbed down to depths respectively on the order of 1, 2, and 3 micrometers, it is necessary to provide, for each photodiode, a photon absorption area, that is, a thickness of substrate 50 under active region 56, with a depth of at least 3 micrometers whatever the type of filter associated with the photodiode.
The current trend is to decrease the lateral dimensions of the image sensors made in monolithic form. When lateral dimensions of the photodiodes become smaller than the absorption depth, that is, approximately 3 micrometers, the phenomena of minority carrier diffusion are no longer negligible and result in a very strong degradation of the image sensor resolution. Indeed, the photons absorbed at the level of the portion of substrate 50 located under active region 56 associated with a given photodiode may cause the forming of electrons which are caught, due to the diffusion, by the photodiode regions 56 adjacent to the given photodiode. This phenomenon, called diffusion video crosstalk, significantly degrades the sensor resolution since electrons originating from photons of a determined wavelength may be captured by a photodiode receiving photons of another wavelength.
Such a disadvantage can be avoided by insulating the portions of substrate 50 located under regions 56 associated with each photodiode. For this purpose, each field insulation region 54 could be extended by a heavily-doped P-type region, which would extend to wafer 52. It would be desirable to be able to form such a region with conventional integrated circuit manufacturing techniques, for example, by implantation steps. However, the width of such a P-type region should be substantially identical to the width of a field insulation region 54, that is, on the order of from 0.4 to 0.5 micrometer. Since such a region should extend down to a 3-micrometer depth, it should be formed by implantation with high energies, greater than 1 Megaelectronvolt. To perform such an implantation, it is then necessary to form resin portions on the monolithic circuit having a thickness greater than 4 micrometers and spaced apart by 0.4 micrometer. This cannot be done with techniques compatible with industrial integrated circuit manufacturing methods.